Pressurized water nuclear reactors generally include a vessel containing the core of the reactor which is immersed in pressurized water for cooling the reactor. The vessel of the reactor, which has a generally cylindrical shape, includes a head which has a hemispherical shape and which can be fitted onto its top.
The head is drilled with holes, at each of which there is fixed by welding a through tubular component constituting an adaptor allowing passage and control of the movement of an extension of a rod cluster for controlling the reactivity of the core or a through passage for a means for measuring inside the reactor, such as a thermocouple column.
The mechanisms for controlling the movement of the core reactivity control rod clusters are fixed to the end parts of each of the adaptors.
Inside each of the tubular through holes of the vessel head there is fixed, in a coaxial position with respect to the tubular through component and with a certain radial clearance, a thermal sleeve which includes a diametrically widened part resting on a diametrically widened support zone situated at the top of the bore of the tubular through component; the thermal sleeve is mounted so as to rotate freely inside the through hole.
The extensions of the nuclear reactor reactivity control rods and the thermocouple columns pass through the vessel head inside the thermal sleeves which are themselves disposed coaxially inside the adaptors of the control rods or more generally inside the tubular through components of the head.
So as to increase the reliability and safety of operation of nuclear reactors and in order to extend the lifetime of these reactors, the operators are led to perform more and more numerous tests of the various elements constituting the nuclear reactor and, if necessary, repairs of defects which may have been detected.
In particular, it maybe necessary to monitor the state of the through components of the vessel head in order to make certain of the integrity of these components after a certain operating time of the reactor, in particular in the zone where these through components are welded to the vessel head.
Defects are detected on the internal surface of a through component, these defects must be repaired, for example by depositing a layer of a metal such as nickel on the internal surface of the through component, in the zone which has the defects, or alternatively by eroding the zone which has defects to a certain depth by machining with removal of material, and refilling this zone with a fault-free replacement metal.
The operations of testing and/or repairing the adaptors of the vessel head of a nuclear reactor must be performed inside the bore of the through component and consequently require demounting of the thermal sleeve, in order to access the internal surface of the bore of the through hole.
These tests and repairs must be carried out during a reactor shutdown, the vessel head being demounted.
The demounting of the thermal sleeve requires complex operations, as the widening of the thermal sleeve resting on a support zone disposed at the top of the adaptor prevents demounting of the sleeve by pulling downwards, on its lower part which is accessible beneath the head.
It is also not possible to demount the sleeve by pulling upwards, as the rod cluster control mechanisms which are fixed by screwing and welding to the tops of the adaptors prevent passage of the thermal sleeve.
It was therefore proposed in French Patent Application No. 9202405, filed in the names of Framatome and Electricite de France on Feb. 28 1992, to test the internal surface of the through component and possibly to repair this component when defects are detected, through a longitudinal slot machined in the thermal sleeve and passing through its wall.
When the repair is made by depositing a layer of nickel on the internal surface of the adaptor, in the zone which has defects, this repair necessarily requires demounting of the thermal sleeve. Furthermore, the electrolytic deposition of the nickel is in itself a long and therefore expensive operation. The length of work on the adaptors of a vessel head for performing the repairs or possibly a preventive treatment by deposition of nickel on the internal surface of the adaptor at the welding zone is of the order of ten days.
Furthermore, the repair or preventive treatment thus performed does not get rid of the cracked metal and the stresses which can lead to cracking underneath the nickel coating.
When the repair is made by eroding and refilling, it may be possible in certain cases to perform these operations through a slot which is machined in the thermal sleeve and used for testing the internal surface of the adaptor. However, no method and device are yet known which allow, simply, quickly, with perfect monitoring and with an excellent surface condition, the surface with defects to be eroded, i.e., the material of the component to be removed to a limited and perfectly monitored depth so as to generate a new fault-free surface.
Neither is a method known which allows simultaneous removal of the material which has defects and stress relief of the component to a certain depth.
More generally, no method is known for machining an internal cylindrical surface of a tubular component, in particular of a metallic component, which is both fast and efficient and which allows a new surface to be obtained which has perfectly defined geometrical and physical characteristics as well as stress relief of the component below the machined surface.